add `manual_slice_fill` lint (#14082)

close #13856

changelog: [`manual_slice_fill`]: new lint

7 files changed, 289 insertions, 34 deletions

diff --git a/tests/ui/manual_memcpy/without_loop_counters.rs b/tests/ui/manual_memcpy/without_loop_counters.rs
index c917fa7f2d0..2f8640cd3f5 100644
--- a/tests/ui/manual_memcpy/without_loop_counters.rs
+++ b/tests/ui/manual_memcpy/without_loop_counters.rs
@@ -1,5 +1,10 @@
 #![warn(clippy::manual_memcpy)]
-#![allow(clippy::assigning_clones, clippy::useless_vec, clippy::needless_range_loop)]
+#![allow(
+    clippy::assigning_clones,
+    clippy::useless_vec,
+    clippy::needless_range_loop,
+    clippy::manual_slice_fill
+)]
 
 //@no-rustfix
 const LOOP_OFFSET: usize = 5000;
diff --git a/tests/ui/manual_memcpy/without_loop_counters.stderr b/tests/ui/manual_memcpy/without_loop_counters.stderr
index 803053b2db2..c881e3fac76 100644
--- a/tests/ui/manual_memcpy/without_loop_counters.stderr
+++ b/tests/ui/manual_memcpy/without_loop_counters.stderr
@@ -1,5 +1,5 @@
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:9:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:14:5
    |
 LL | /     for i in 0..src.len() {
 LL | |
@@ -12,7 +12,7 @@ LL | |     }
    = help: to override `-D warnings` add `#[allow(clippy::manual_memcpy)]`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:16:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:21:5
    |
 LL | /     for i in 0..src.len() {
 LL | |
@@ -21,7 +21,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst[10..(src.len() + 10)].copy_from_slice(&src[..]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:22:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:27:5
    |
 LL | /     for i in 0..src.len() {
 LL | |
@@ -30,7 +30,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst[..src.len()].copy_from_slice(&src[10..(src.len() + 10)]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:28:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:33:5
    |
 LL | /     for i in 11..src.len() {
 LL | |
@@ -39,7 +39,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst[11..src.len()].copy_from_slice(&src[(11 - 10)..(src.len() - 10)]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:34:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:39:5
    |
 LL | /     for i in 0..dst.len() {
 LL | |
@@ -48,7 +48,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst.copy_from_slice(&src[..dst.len()]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:48:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:53:5
    |
 LL | /     for i in 10..256 {
 LL | |
@@ -64,7 +64,7 @@ LL +     dst2[(10 + 500)..(256 + 500)].copy_from_slice(&src[10..256]);
    |
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:61:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:66:5
    |
 LL | /     for i in 10..LOOP_OFFSET {
 LL | |
@@ -73,7 +73,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst[(10 + LOOP_OFFSET)..(LOOP_OFFSET + LOOP_OFFSET)].copy_from_slice(&src[(10 - some_var)..(LOOP_OFFSET - some_var)]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:75:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:80:5
    |
 LL | /     for i in 0..src_vec.len() {
 LL | |
@@ -82,7 +82,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst_vec[..src_vec.len()].copy_from_slice(&src_vec[..]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:105:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:110:5
    |
 LL | /     for i in from..from + src.len() {
 LL | |
@@ -91,7 +91,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst[from..(from + src.len())].copy_from_slice(&src[..(from + src.len() - from)]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:110:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:115:5
    |
 LL | /     for i in from..from + 3 {
 LL | |
@@ -100,7 +100,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst[from..(from + 3)].copy_from_slice(&src[..(from + 3 - from)]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:116:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:121:5
    |
 LL | /     for i in 0..5 {
 LL | |
@@ -109,7 +109,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst[..5].copy_from_slice(&src);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:122:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:127:5
    |
 LL | /     for i in 0..0 {
 LL | |
@@ -118,7 +118,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst[..0].copy_from_slice(&src[..0]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:146:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:151:5
    |
 LL | /     for i in 0..4 {
 LL | |
@@ -127,7 +127,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst.copy_from_slice(&src[..4]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:152:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:157:5
    |
 LL | /     for i in 0..5 {
 LL | |
@@ -136,7 +136,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst[..5].copy_from_slice(&src);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:158:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:163:5
    |
 LL | /     for i in 0..5 {
 LL | |
@@ -145,7 +145,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst.copy_from_slice(&src);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:205:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:210:5
    |
 LL | /     for i in 0..5 {
 LL | |
@@ -154,7 +154,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst.copy_from_slice(&src[0]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:211:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:216:5
    |
 LL | /     for i in 0..5 {
 LL | |
@@ -163,7 +163,7 @@ LL | |     }
    | |_____^ help: try replacing the loop by: `dst.copy_from_slice(&src[0][1]);`
 
 error: it looks like you're manually copying between slices
-  --> tests/ui/manual_memcpy/without_loop_counters.rs:219:5
+  --> tests/ui/manual_memcpy/without_loop_counters.rs:224:5
    |
 LL | /     for i in 0..src.len() {
 LL | |
diff --git a/tests/ui/manual_slice_fill.fixed b/tests/ui/manual_slice_fill.fixed
new file mode 100644
index 00000000000..397a156a2dc
--- /dev/null
+++ b/tests/ui/manual_slice_fill.fixed
@@ -0,0 +1,101 @@
+#![warn(clippy::manual_slice_fill)]
+#![allow(clippy::needless_range_loop)]
+
+macro_rules! assign_element {
+    ($slice:ident, $index:expr) => {
+        $slice[$index] = 0;
+    };
+}
+
+macro_rules! assign_element_2 {
+    ($i:expr) => {
+        $i = 0;
+    };
+}
+
+struct NoClone;
+
+fn num() -> usize {
+    5
+}
+
+fn should_lint() {
+    let mut some_slice = [1, 2, 3, 4, 5];
+
+    some_slice.fill(0);
+
+    let x = 5;
+    some_slice.fill(x);
+
+    some_slice.fill(0);
+
+    // This should trigger `manual_slice_fill`, but the applicability is `MaybeIncorrect` since comments
+    // within the loop might be purely informational.
+    some_slice.fill(0);
+}
+
+fn should_not_lint() {
+    let mut some_slice = [1, 2, 3, 4, 5];
+
+    // Should not lint because we can't determine if the scope of the loop is intended to access all the
+    // elements of the slice.
+    for i in 0..5 {
+        some_slice[i] = 0;
+    }
+
+    // Should not lint, as using a function to assign values to elements might be
+    // intentional. For example, the contents of `num()` could be temporary and subject to change
+    // later.
+    for i in 0..some_slice.len() {
+        some_slice[i] = num();
+    }
+
+    // Should not lint because this loop isn't equivalent to `fill`.
+    for i in 0..some_slice.len() {
+        some_slice[i] = 0;
+        println!("foo");
+    }
+
+    // Should not lint because it may be intentional to use a macro to perform an operation equivalent
+    // to `fill`.
+    for i in 0..some_slice.len() {
+        assign_element!(some_slice, i);
+    }
+
+    let another_slice = [1, 2, 3];
+    // Should not lint because the range is not for `some_slice`.
+    for i in 0..another_slice.len() {
+        some_slice[i] = 0;
+    }
+
+    let mut vec: Vec<Option<NoClone>> = Vec::with_capacity(5);
+    // Should not lint because `NoClone` does not have `Clone` trait.
+    for i in 0..vec.len() {
+        vec[i] = None;
+    }
+
+    // Should not lint, as using a function to assign values to elements might be
+    // intentional. For example, the contents of `num()` could be temporary and subject to change
+    // later.
+    for i in &mut some_slice {
+        *i = num();
+    }
+
+    // Should not lint because this loop isn't equivalent to `fill`.
+    for i in &mut some_slice {
+        *i = 0;
+        println!("foo");
+    }
+
+    // Should not lint because it may be intentional to use a macro to perform an operation equivalent
+    // to `fill`.
+    for i in &mut some_slice {
+        assign_element_2!(*i);
+    }
+
+    let mut vec: Vec<Option<NoClone>> = Vec::with_capacity(5);
+    // Should not lint because `NoClone` does not have `Clone` trait.
+    for i in &mut vec {
+        *i = None;
+    }
+}
diff --git a/tests/ui/manual_slice_fill.rs b/tests/ui/manual_slice_fill.rs
new file mode 100644
index 00000000000..c25127ca613
--- /dev/null
+++ b/tests/ui/manual_slice_fill.rs
@@ -0,0 +1,110 @@
+#![warn(clippy::manual_slice_fill)]
+#![allow(clippy::needless_range_loop)]
+
+macro_rules! assign_element {
+    ($slice:ident, $index:expr) => {
+        $slice[$index] = 0;
+    };
+}
+
+macro_rules! assign_element_2 {
+    ($i:expr) => {
+        $i = 0;
+    };
+}
+
+struct NoClone;
+
+fn num() -> usize {
+    5
+}
+
+fn should_lint() {
+    let mut some_slice = [1, 2, 3, 4, 5];
+
+    for i in 0..some_slice.len() {
+        some_slice[i] = 0;
+    }
+
+    let x = 5;
+    for i in 0..some_slice.len() {
+        some_slice[i] = x;
+    }
+
+    for i in &mut some_slice {
+        *i = 0;
+    }
+
+    // This should trigger `manual_slice_fill`, but the applicability is `MaybeIncorrect` since comments
+    // within the loop might be purely informational.
+    for i in 0..some_slice.len() {
+        some_slice[i] = 0;
+        // foo
+    }
+}
+
+fn should_not_lint() {
+    let mut some_slice = [1, 2, 3, 4, 5];
+
+    // Should not lint because we can't determine if the scope of the loop is intended to access all the
+    // elements of the slice.
+    for i in 0..5 {
+        some_slice[i] = 0;
+    }
+
+    // Should not lint, as using a function to assign values to elements might be
+    // intentional. For example, the contents of `num()` could be temporary and subject to change
+    // later.
+    for i in 0..some_slice.len() {
+        some_slice[i] = num();
+    }
+
+    // Should not lint because this loop isn't equivalent to `fill`.
+    for i in 0..some_slice.len() {
+        some_slice[i] = 0;
+        println!("foo");
+    }
+
+    // Should not lint because it may be intentional to use a macro to perform an operation equivalent
+    // to `fill`.
+    for i in 0..some_slice.len() {
+        assign_element!(some_slice, i);
+    }
+
+    let another_slice = [1, 2, 3];
+    // Should not lint because the range is not for `some_slice`.
+    for i in 0..another_slice.len() {
+        some_slice[i] = 0;
+    }
+
+    let mut vec: Vec<Option<NoClone>> = Vec::with_capacity(5);
+    // Should not lint because `NoClone` does not have `Clone` trait.
+    for i in 0..vec.len() {
+        vec[i] = None;
+    }
+
+    // Should not lint, as using a function to assign values to elements might be
+    // intentional. For example, the contents of `num()` could be temporary and subject to change
+    // later.
+    for i in &mut some_slice {
+        *i = num();
+    }
+
+    // Should not lint because this loop isn't equivalent to `fill`.
+    for i in &mut some_slice {
+        *i = 0;
+        println!("foo");
+    }
+
+    // Should not lint because it may be intentional to use a macro to perform an operation equivalent
+    // to `fill`.
+    for i in &mut some_slice {
+        assign_element_2!(*i);
+    }
+
+    let mut vec: Vec<Option<NoClone>> = Vec::with_capacity(5);
+    // Should not lint because `NoClone` does not have `Clone` trait.
+    for i in &mut vec {
+        *i = None;
+    }
+}
diff --git a/tests/ui/manual_slice_fill.stderr b/tests/ui/manual_slice_fill.stderr
new file mode 100644
index 00000000000..3aa980f6919
--- /dev/null
+++ b/tests/ui/manual_slice_fill.stderr
@@ -0,0 +1,38 @@
+error: manually filling a slice
+  --> tests/ui/manual_slice_fill.rs:25:5
+   |
+LL | /     for i in 0..some_slice.len() {
+LL | |         some_slice[i] = 0;
+LL | |     }
+   | |_____^ help: try: `some_slice.fill(0);`
+   |
+   = note: `-D clippy::manual-slice-fill` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::manual_slice_fill)]`
+
+error: manually filling a slice
+  --> tests/ui/manual_slice_fill.rs:30:5
+   |
+LL | /     for i in 0..some_slice.len() {
+LL | |         some_slice[i] = x;
+LL | |     }
+   | |_____^ help: try: `some_slice.fill(x);`
+
+error: manually filling a slice
+  --> tests/ui/manual_slice_fill.rs:34:5
+   |
+LL | /     for i in &mut some_slice {
+LL | |         *i = 0;
+LL | |     }
+   | |_____^ help: try: `some_slice.fill(0);`
+
+error: manually filling a slice
+  --> tests/ui/manual_slice_fill.rs:40:5
+   |
+LL | /     for i in 0..some_slice.len() {
+LL | |         some_slice[i] = 0;
+LL | |         // foo
+LL | |     }
+   | |_____^ help: try: `some_slice.fill(0);`
+
+error: aborting due to 4 previous errors
+
diff --git a/tests/ui/needless_range_loop.rs b/tests/ui/needless_range_loop.rs
index 3f242195330..75f1896eded 100644
--- a/tests/ui/needless_range_loop.rs
+++ b/tests/ui/needless_range_loop.rs
@@ -2,7 +2,8 @@
 #![allow(
     clippy::uninlined_format_args,
     clippy::unnecessary_literal_unwrap,
-    clippy::useless_vec
+    clippy::useless_vec,
+    clippy::manual_slice_fill
 )]
 //@no-rustfix
 static STATIC: [usize; 4] = [0, 1, 8, 16];
diff --git a/tests/ui/needless_range_loop.stderr b/tests/ui/needless_range_loop.stderr
index dc2cf437e02..503d796e5e8 100644
--- a/tests/ui/needless_range_loop.stderr
+++ b/tests/ui/needless_range_loop.stderr
@@ -1,5 +1,5 @@
 error: the loop variable `i` is only used to index `vec`
-  --> tests/ui/needless_range_loop.rs:15:14
+  --> tests/ui/needless_range_loop.rs:16:14
    |
 LL |     for i in 0..vec.len() {
    |              ^^^^^^^^^^^^
@@ -12,7 +12,7 @@ LL |     for <item> in &vec {
    |         ~~~~~~    ~~~~
 
 error: the loop variable `i` is only used to index `vec`
-  --> tests/ui/needless_range_loop.rs:26:14
+  --> tests/ui/needless_range_loop.rs:27:14
    |
 LL |     for i in 0..vec.len() {
    |              ^^^^^^^^^^^^
@@ -23,7 +23,7 @@ LL |     for <item> in &vec {
    |         ~~~~~~    ~~~~
 
 error: the loop variable `j` is only used to index `STATIC`
-  --> tests/ui/needless_range_loop.rs:32:14
+  --> tests/ui/needless_range_loop.rs:33:14
    |
 LL |     for j in 0..4 {
    |              ^^^^
@@ -34,7 +34,7 @@ LL |     for <item> in &STATIC {
    |         ~~~~~~    ~~~~~~~
 
 error: the loop variable `j` is only used to index `CONST`
-  --> tests/ui/needless_range_loop.rs:37:14
+  --> tests/ui/needless_range_loop.rs:38:14
    |
 LL |     for j in 0..4 {
    |              ^^^^
@@ -45,7 +45,7 @@ LL |     for <item> in &CONST {
    |         ~~~~~~    ~~~~~~
 
 error: the loop variable `i` is used to index `vec`
-  --> tests/ui/needless_range_loop.rs:42:14
+  --> tests/ui/needless_range_loop.rs:43:14
    |
 LL |     for i in 0..vec.len() {
    |              ^^^^^^^^^^^^
@@ -56,7 +56,7 @@ LL |     for (i, <item>) in vec.iter().enumerate() {
    |         ~~~~~~~~~~~    ~~~~~~~~~~~~~~~~~~~~~~
 
 error: the loop variable `i` is only used to index `vec2`
-  --> tests/ui/needless_range_loop.rs:51:14
+  --> tests/ui/needless_range_loop.rs:52:14
    |
 LL |     for i in 0..vec.len() {
    |              ^^^^^^^^^^^^
@@ -67,7 +67,7 @@ LL |     for <item> in vec2.iter().take(vec.len()) {
    |         ~~~~~~    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: the loop variable `i` is only used to index `vec`
-  --> tests/ui/needless_range_loop.rs:56:14
+  --> tests/ui/needless_range_loop.rs:57:14
    |
 LL |     for i in 5..vec.len() {
    |              ^^^^^^^^^^^^
@@ -78,7 +78,7 @@ LL |     for <item> in vec.iter().skip(5) {
    |         ~~~~~~    ~~~~~~~~~~~~~~~~~~
 
 error: the loop variable `i` is only used to index `vec`
-  --> tests/ui/needless_range_loop.rs:61:14
+  --> tests/ui/needless_range_loop.rs:62:14
    |
 LL |     for i in 0..MAX_LEN {
    |              ^^^^^^^^^^
@@ -89,7 +89,7 @@ LL |     for <item> in vec.iter().take(MAX_LEN) {
    |         ~~~~~~    ~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: the loop variable `i` is only used to index `vec`
-  --> tests/ui/needless_range_loop.rs:66:14
+  --> tests/ui/needless_range_loop.rs:67:14
    |
 LL |     for i in 0..=MAX_LEN {
    |              ^^^^^^^^^^^
@@ -100,7 +100,7 @@ LL |     for <item> in vec.iter().take(MAX_LEN + 1) {
    |         ~~~~~~    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: the loop variable `i` is only used to index `vec`
-  --> tests/ui/needless_range_loop.rs:71:14
+  --> tests/ui/needless_range_loop.rs:72:14
    |
 LL |     for i in 5..10 {
    |              ^^^^^
@@ -111,7 +111,7 @@ LL |     for <item> in vec.iter().take(10).skip(5) {
    |         ~~~~~~    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: the loop variable `i` is only used to index `vec`
-  --> tests/ui/needless_range_loop.rs:76:14
+  --> tests/ui/needless_range_loop.rs:77:14
    |
 LL |     for i in 5..=10 {
    |              ^^^^^^
@@ -122,7 +122,7 @@ LL |     for <item> in vec.iter().take(10 + 1).skip(5) {
    |         ~~~~~~    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: the loop variable `i` is used to index `vec`
-  --> tests/ui/needless_range_loop.rs:81:14
+  --> tests/ui/needless_range_loop.rs:82:14
    |
 LL |     for i in 5..vec.len() {
    |              ^^^^^^^^^^^^
@@ -133,7 +133,7 @@ LL |     for (i, <item>) in vec.iter().enumerate().skip(5) {
    |         ~~~~~~~~~~~    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: the loop variable `i` is used to index `vec`
-  --> tests/ui/needless_range_loop.rs:86:14
+  --> tests/ui/needless_range_loop.rs:87:14
    |
 LL |     for i in 5..10 {
    |              ^^^^^
@@ -144,7 +144,7 @@ LL |     for (i, <item>) in vec.iter().enumerate().take(10).skip(5) {
    |         ~~~~~~~~~~~    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: the loop variable `i` is used to index `vec`
-  --> tests/ui/needless_range_loop.rs:92:14
+  --> tests/ui/needless_range_loop.rs:93:14
    |
 LL |     for i in 0..vec.len() {
    |              ^^^^^^^^^^^^